1. Field of the Invention
The present invention relates to an exhaust system for a motorcycle and, more particularly, to an exhaust system for a motorcycle provided with a multicylinder engine.
2. Description of the Related Art
FIG. 8 shows an exhaust system, which is disclosed in JP-A 2002-138828, for a motorcycle provided with a four-cylinder engine. As shown in FIG. 8, the exhaust system is formed by connecting four upstream exhaust pipes 101 connected to the exhaust ports of the four cylinders to two exhaust aggregating members 102, connecting the two exhaust aggregating members 102 to two intermediate exhaust pipes 103, connecting the two intermediate exhaust pipes 103 to a downstream exhaust aggregating member 104, connecting the downstream exhaust aggregating member to a downstream exhaust pipe 105, and connecting the downstream exhaust pipe 105 to a single exhaust muffler. The exhaust system is provided with an exhaust control valve 106 to improve the output characteristic of the engine and to reduce exhaust noise. In some exhaust system, adjacent exhaust pipes are connected by a connecting pipe. The exhaust control valve 106 is capable of changing the sectional area of an exhaust passage of the exhaust system. The exhaust control valve 106 changes the sectional area of the exhaust passage according to the change of the operating condition of the engine, such as the change of the load on the engine, change of fuel injection rate, the change of running speed or the change of engine speed to improve the output characteristic of the engine and to reduce exhaust noise.
In some cases, the exhaust control valve 106 is incorporated into each of the exhaust aggregating members 102 which are connected to the four upstream exhaust pipes 101. In the exhaust system shown in FIG. 8, the exhaust control valve 106 is incorporated into the downstream exhaust pipe 105 which is positioned on the downstream side with respect to the downstream exhaust aggregating member 104.
FIG. 9 shows an exhaust system, which is disclosed in JP-A 63-16127, for a motorcycle provided with a four-cylinder engine. As shown in FIG. 9, this exhaust system is formed by connecting four exhaust pipes 101 connected to the exhaust ports of the cylinders to a single exhaust aggregating member 111, connecting the exhaust aggregating member 111 to a single exhaust pipe 112, and incorporating an exhaust control valve 113 into the exhaust aggregating member 111.
Moreover, in an exhaust system of a related art, as shown in FIG. 10, exhaust passages in two adjacent exhaust pipes 200 of the exhaust system are connected by a connecting pipe 201 having the shape of a circular arc and joined to the adjacent exhaust pipes 200 by welding.
In the exhaust system shown in FIG. 8, an exhaust passage between the exhaust port of the engine and the exhaust control valve 106 becomes inevitably long. Consequently, the response of the exhaust control valve 106 to the change of the operating condition of the engine is delayed, and hence the exhaust control valve 106 is unable to contribute effectively to the improvement of the output characteristic of the engine.
Since the exhaust system shown in FIG. 9 has the four exhaust pipes 101 connected to the single exhaust aggregating member 111, the exhaust aggregating member 111 becomes inevitably large, thereby the exhaust control valve 113 becomes large and has complicated construction inevitably. The large exhaust control valve 113 places restrictions on the layout of the exhaust system and increases the cost of the exhaust system.
When the connecting pipe 201 having the shape of a circular arc is joined to the two adjacent exhaust pipes 200 to connect the exhaust passages in the two adjacent exhaust pipes 200 as shown in FIG. 10, the two adjacent exhaust pipes 200 must be spaced a fixed distance apart to prevent the collision of the exhaust pipes 200 against each other when shocks are exerted thereon. Further, a space of A1×A2 is necessary to install the arc shaped connecting pipe 201, which places a restriction on the layout of the exhaust system.